A: The answer depends on what you’re trying to do and what type of plug-in you’re using. There are no hard and fast rules, but here are a few guidelines to get you started:
As a general rule, if 100% of a single signal is to be processed by the plug-in, then an insert is the way to go. This is typically how compressors, equalizers, gates, de-essers, limiters, and similar plug-ins are used.
If more than one signal is to be routed through the plug-in, then an aux return is the appropriate choice.
If an effect is being added that becomes an integral part of a sound, such as a rhythmic delay, a chorus that becomes part of the timbre, and so on, then using the plug-in as an insert may help keep things organized.